Passive Heat Exchanger Comprising Thermally Conductive Foam

ABSTRACT

This disclosure is directed to devices, systems, and methods associated with a passive heat exchanger using thermally conductive foam. With reference to some systems the thermally conductive foam is positioned between a heat source and a heat dissipater to lower an operating temperature. With reference to other systems the thermally conductive foam is positioned between a component to be heated and a heat source to increase an operating temperature. Some embodiments of the thermally conductive foam are substantially pliable, other embodiments of the thermally conductive foam are substantially rigid, and other embodiments of the thermally conductive foam are initially fluid. Some devices related to the present disclosure include thermally conductive foam having thermally conductive particles within the thermally conductive foam.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of co-pending U.S. Provisional Patent Application No. 61/343,677, filed May 3, 2010, by the same applicant as this application, and which is expressly incorporated herein, in its entirety, be reference.

BACKGROUND

Carbon or graphite foam derived from a pitch precursor is an open-celled and porous composition, which may include as much as 20,000 square inches of surface area per cubic inch of material. Some methods of producing carbon foams include a blowing technique wherein the pitch precursor is melted and passed from a high pressure region to a low pressure region. This technique causes the low molecular weight compounds in the pitch to vaporize, which results in a pitch foam. The pitch foam is then oxidatively stabilized by heating in air to set the pitch so that it does not melt during carbonization. The oxidized pitch may then be carbonized to produce a graphitic carbon foam.

Other methods of producing carbon foam include a polymeric precursor mixed with the pitch. High pressure and heat is applied to the pitch until a specified temperature is achieved when the pressure is released, which causes the pitch to foam as volatile compounds are released. The polymeric precursors are cured and then carbonized without a stabilization step. Once the foam is formed it is then typically bonded to a facesheet.

The machining of thermally conductive carbon or graphitic foam into various articles is difficult. In the case of a heat sink comprising thermally conductive foam, one method of improving heat transfer is to configure the thermally conductive foam to include very small fin structures. As molding features such as fins into thermally conductive foam has not yet proven practical, thermally conductive foam blanks may be formed and then machined to include the desired features. Even so, the act of machining small features in thermally conductive foam is challenging and often results in fracture of the fins due to the brittle nature of most thermally conductive carbon or graphitic foams.

Improvements to the present state of thermally conductive foam technology are desirable. Particularly desirable are improvements to the present state of the use of thermally conductive carbon or graphitic foams to passively cool and to passively heat.

SUMMARY

One embodiment of the present disclosure is a passive heat exchanger system that cools or otherwise lowers the temperature of a heat generating source. Here the passive heat exchanger system includes a heat generating component and a near-by heat dissipater. Between the heat generating component and the heat dissipater is positioned a thermally conductive foam. This thermally conductive foam is configured to transfer heat from the heat generating component to the heat dissipater. In another embodiment a heat sink is attached to the heat dissipater and is configured to transfer heat from the heat dissipater to the atmosphere. The open-cell structure and porous nature of thermally conductive foam typically defines voids throughout the conduit manufactured by the thermally conductive foam. In another embodiment the voids along one surface of the thermally conductive foam are substantially filled with thermally conductive particles. In some embodiments the thermally conductive foam is substantially pliable and in other embodiments it is substantially rigid. In still another embodiment, the thermally conductive foam is formed from a compound of thermally conductive foam powder, adhesive, and solvent, which is applied as substantially a liquid or paste and cures to a solid state.

Another embodiment of the present disclosure is a typical street light having a passive heat exchanger system. Here the street light has a heat dissipater in the form of a street light housing, and within the street light housing is located a heat generating component in the form of a ballast or generator which creates heat while it operates to power the street light lamp(s). Between the generator and the street light housing is positioned a thermally conductive foam. This thermally conductive foam is in thermal communication with the generator and with a side or surface of the street light housing. This thermally conductive foam is configured to transfer heat from the generator to the street light housing, thereby reducing the operating temperature or otherwise cooling the generator. In one embodiment the street light housing is made of a metallic material such as aluminum, and in another embodiment the street light housing is made of a polymer. In either embodiment a heat sink may be attached to the street light housing and further transfer heat from the street light housing to the atmosphere.

Additional embodiments of the present disclosure include methods for transferring heat with a passive heat exchanger to cool or otherwise lower the operating temperature of a heat generating component, source, device, or system. This method begins with identifying at least one heat source to be cooled. One side or end of a thermally conductive foam is then attached to the heat source and the other side or end is attached to a heat dissipater. In various embodiments the heat dissipater is any type and configuration of thermally conductive material to which heat may be transferred. With the thermally conductive foam in thermal communication with a heat source and a heat dissipater, the thermally conductive foam is configured to transfer heat from the heat source to the heat dissipater. The thermally conductive foam is then allowed to transfer heat from the heat source to the heat dissipater. Another embodiment includes positioning a heat sink in thermal communication with the heat dissipater to further dissipate heat from the thermally conductive foam. Another embodiment includes providing a thermally conductive foam having voids with thermally conductive particles positioned substantially throughout the one side of the thermally conductive foam. Still another embodiment includes providing a thermally conductive foam made from a compound of thermally conductive foam powder, adhesive, and solvent.

Additional embodiments of the present disclosure include methods for transferring heat with a passive heat exchanger to heat or otherwise raise the operating temperature of a component, device, or system. This method begins with identifying at least one component to be heated. One side or end of a thermally conductive foam is then attached to the component to be heated and the other side or end is placed in thermal communication with a heat source. With the thermally conductive foam in thermal communication with a component to be heated and a heat source, the thermally conductive foam is configured to transfer heat from the heat source to the component to be heated. The thermally conductive foam is then allowed to transfer heat from the heat source to the component to be heated. Another embodiment includes providing a thermally conductive foam having voids with thermally conductive particles positioned substantially throughout the one side of the thermally conductive foam. Still another embodiment includes providing a thermally conductive foam made from a compound of thermally conductive foam powder, adhesive, and solvent.

Alternative and additional embodiments of a passive heat exchanger include a thermally conductive foam that is applied in a substantially liquid or paste form to a heat source, or a component to be heated, or a heat dissipater, and cures to substantially solid form. In one embodiment the thermally conductive foam is manufactured from substantially equal parts of thermally conductive foam powder, adhesive, and solvent. In another embodiment the thermally conductive foam powder is primarily carbon while in another it is primarily graphite. In another embodiment the adhesive is epoxy. In another embodiment the solvent is acetone.

The features, functions, and advantages that have been discussed herein can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments, further details of which can be seen with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary passive heat exchanger configured to cool or lower an operating temperature.

FIG. 2 is a chart illustrating the results of a comparative test plotting temperature/time of an embodiment of the present disclosure.

FIG. 3 is a chart illustrating the results of a comparative test plotting temperature/time of an embodiment of the present disclosure.

FIG. 4 illustrates an alternative exemplary passive heat exchanger configured to cool or lower an operating temperature.

FIG. 5 is a table illustrating the results of a comparative test plotting temperature changes of embodiments of the present disclosure.

FIG. 6 illustrates an exemplary passive heat exchanger configured to heat or raise an operating temperature.

FIG. 7 is a table illustrating performance characteristics of embodiments of the present disclosure.

FIG. 8 is a table illustrating performance characteristics of embodiments of the present disclosure.

DETAILED DESCRIPTION

In many closed or restricted environments it may be desirable to reduce the heat or otherwise cool the heat sources present in such environments. Turning now to FIG. 1, there is illustrated an exemplary passive heat exchanger according to the present disclosure applied to a system that includes a heat source and a heat dissipating structure or heat dissipating layer. For purposes of the present teaching and not limitation or restriction, here the terms “heat dissipating structure” and “heat dissipating layer” are referred together as “heat dissipater”. More specifically, FIG. 1 shows a heat dissipating structure in the form of a typical die cast aluminum street light housing 100 in the so-called ‘cobra head’ style, which the present disclosure teaches may be configured to be a more effective heat dissipating structure. The illustrated embodiment comprises an electrically powered heat source in the form of a ballast or generator 102. In this exemplary embodiment the heat source to be cooled is the generator 102, which in a typical configuration has a base that measures four inches (4″) in width by six inches (6″) in length. To the base of the generator 102 is attached a thermally conductive foam conduit 104 measuring one-eighth inch (⅛″) thick by four inches (4″) in width by six inches (6″) in length. In alternative embodiments other dimensions of thermally conductive foam conduit 104 of various thicknesses, including less than one-eighth inch (⅛″) and greater than one-eighth inch (⅛″), are contemplated and included in those embodiments.

The thermally conductive foam conduit 104 is then attached to a first or inner side 106 of the street light housing 100. In an exemplary embodiment the thermally conductive foam conduit 104 is bonded with a heat conducting epoxy to the inner side 106 of the street light housing 100, and the base of the generator 102 is bonded with a heat conducting epoxy to the thermally conductive foam conduit 104, such that the thermally conductive foam conduit 104 is in thermal communication with the generator 102 and the inner side 106 of the street light housing 100. By way of teaching and not limitation, the heat conducting epoxy may be a thermal interface material in the form of liquid adhesive, tapes, pads, sprays, combinations thereof, and the like. To a second or outer side 108 of the street light housing 100 may be located an auxiliary heat sink 110.

In various embodiments the thermally conductive foam that comprises the thermally conductive foam conduit 104 has a thermal conductivity of at least approximately 40 W/mK, while in alternative embodiments the thermal conductivity is greater and in other embodiments the thermal conductivity is less. In various embodiments the thermally conductive foam has a surface area of at least approximately 6,000 m²/m³, while in alternative embodiments the surface area is greater and in other embodiments the surface area is less. In various embodiments the thermally conductive foam is characterized by an x-ray diffraction pattern having doublet 100 and 101 peaks characterized by a relative peak split factor of not greater than approximately 0.470, while in alternative embodiments the x-ray diffraction pattern is greater and in other embodiments the x-ray diffraction pattern is less. In various embodiments the thermally conductive foam comprises substantially ellipsoidal pores with a mean pore diameter of approximately not greater than 340 micron, while in alternative embodiments the mean pore diameter is greater and in other embodiments the mean pore diameter is less. In various embodiments the thermally conductive foam is graphitic and exhibits substantially isotropic thermal conductivity. Alternative embodiments include thermally conductive foam conduit(s) 104 manufactured of carbon or graphite foam having the characteristics described herein, combinations thereof, and the like.

The street light housing 100 is in thermal communication with the thermally conductive foam conduit 104 from the inner side 106 and ambient air from the outer side 108. Heat created by the generator 102 is transferred through its base to the thermally conductive foam conduit 104 and from the thermally conductive foam conduit 104 to the street light housing 100, which in the illustrated embodiment is constructed primarily of aluminum. The heat from the outer side 108 is then dissipated to the ambient air or atmosphere. An advantage of the illustrated embodiment according to the present disclosure is that the thermally conductive foam conduit 104 can remove heat from the generator 102 faster than the heat dissipating structure, that is, the street light housing 100, by itself. Accordingly, together with the thermally conductive foam conduit 104, the street light housing 100 constructed primarily of aluminum can dissipate heat faster and maintain a lower operating temperature than the street light housing 100 without the thermally conductive foam conduit 104.

In the illustrated embodiment the street light housing 100, constructed primarily of aluminum, is itself a heat sink for the heat created by the generator 102. The illustrated embodiment includes an auxiliary heat sink 110, in the form of a fin configured to increase the surface area of the street light housing 100 in contact with the ambient air, in order to increase the heat dissipated to the ambient air or atmosphere. In alternative embodiments an auxiliary heat sink 110 may be attached to an existing street light housing 100, such as in the case of a retro-fitted application of the present disclosure that includes an auxiliary heat sink 110. In still other alternative embodiments no auxiliary heat sink 110 is attached, such as in the case of a retro-fitted application of the present disclosure that does not include an auxiliary heat sink 110.

For the present purposes of teaching and illustration, and not restriction or limitation, the heat source to be cooled in the exemplary embodiment shown in FIG. 1 is an electrically powered ballast or generator 102 and the heat dissipater is a street light housing 100. An alternative embodiment includes one or more passive heat exchangers for an LED streetlight. In that example, a thermally conductive foam conduit 104 may be positioned between and in thermal communication with an LED board and street light housing 100, and/or between and in thermal communication with the LED generator and street light housing 100. Further, in that example, the LED street light housing 100 is manufactured of aluminum, plastic, or any other material that may absorb and dissipate heat. Still other embodiments include a thermally conductive foam conduit 104 in thermal communication with a heat source and a heat dissipating structure, such as but not limited to, a motor controller within a motor controller cabinet; a computer component within a computer casing; an air-handling motor within a plenum; an appliance component mounted to a heat dissipating platform; combinations thereof, and the like. Alternative embodiments include heat dissipating structures comprising insulative polymers, conductive metals, combinations thereof, and the like.

FIG. 2 is a chart that illustrates the results of a temperature/time comparative test that includes a passive heat exchanger according to the present disclosure. In this test two substantially identical cobra head style induction lamp street lights, having a die cast aluminum housing and induction lamp with ballast or generator, were provided. One street light was stock, that is, not modified or otherwise altered. In FIG. 2 this street light is designated “Cobra Stock”. The other street light was modified according to the present disclosure to include a carbon foam conduit 104 positioned between the generator 102 and inner side 106 of the housing 100, as shown in FIG. 1. In FIG. 2 this street light is designated “Cobra Modified”.

As illustrated in FIG. 2 and with continued reference to FIG. 1, the generator 102 operating temperature of the stock or unmodified street light rose to 147 degrees Fahrenheit (147° F.) within two (2) hours of operation. In contrast, the generator 102 operating temperature of the street light modified to include the carbon foam conduit 104 rose to only 115 degrees Fahrenheit (115° F.) within the same two (2) hours. This comparative test indicates the combination of a carbon foam conduit 104 in thermal communication with an aluminum heat sink such as the street light housing 100, removes and dissipates heat more effectively than an aluminum heat sink without the carbon foam conduit 104.

Members of the induction light industry have reported that a generator 102 operating temperature of 149 degrees Fahrenheit (149° F.) results in a ballast failure rate of fifty-percent (50%) after 100,000 hours. Members of the induction light industry have estimated that an eighteen degree Fahrenheit (18° F.) reduction in the operating temperature of a generator 102 will double the life expectancy of the generator 102. As shown by FIG. 2, the generator 102 modified according to the present disclosure realized a thirty-two degree Fahrenheit (32° F.) reduction in the operating temperature. Accordingly, an advantage taught by the present disclosure is that a thermally conductive foam conduit can effectively dissipate heat, thereby extending the useful life of the device or component. Another advantage taught by the present disclosure is that the use of a thermally conductive foam conduit to dissipate heat can reduce the repair or replacement costs of a component, device, or system.

FIG. 3 is a chart that illustrates the results of a temperature/time comparative test that includes a passive heat exchanger according to the present disclosure. In this test two substantially identical cobra head style induction lamp street lights, having a die cast aluminum housing and induction lamp with ballast or generator, were provided. With reference also to FIG. 1, one street light was modified to include an aluminum plate as a conduit positioned between the generator 102 and inner side 106 of the housing 100. The aluminum plate conduit measured one-eighth inch (0.125″) thick by four inches (4″) in width by six inches (6″) in length, and was bonded using heat conducting epoxy on one side to the base of the generator 102 and on the other side to the inner side 106 of the housing 100. In FIG. 3 this street light is designated “Aluminum Conduit”. The other street light was modified according to the present disclosure to include a carbon foam conduit 104 positioned between the generator 102 and inner side 106 of the housing 100, as shown in FIG. 1. In FIG. 3 this street light is designated “Carbon Conduit”.

As illustrated in FIG. 3, the generator operating temperature of the Aluminum Conduit street light rose to 115 degrees Fahrenheit (115° F.) within 200 minutes of operation. In contrast, the generator operating temperature of the street light modified to include the carbon foam conduit rose to only 100 degrees Fahrenheit (100° F.) within the same 200 minutes. This comparative test indicates the combination of a carbon foam conduit in thermal communication with a metallic heat sink, such as the aluminum street light housing, removes and dissipates heat more effectively than a metallic heat conduit such as the aluminum plate in thermal communication with a metallic heat sink such as the aluminum street light housing. Accordingly, an advantage taught by the present disclosure is that a carbon foam conduit can dissipate heat more effectively than conventional conductive heat conduits, such as an aluminum plate. Another advantage taught by the present disclosure is that a carbon foam conduit in combination with a metallic heat sink can more efficiently remove heat than a metallic conduit in combination with a metallic heat sink.

Turning now to FIG. 4, there is illustrated another exemplary passive heat exchanger according to the present disclosure, applied to a system that includes a heat source and a heat dissipater. More specifically, the illustrated embodiment comprises a heat dissipater in the form of a typical die cast aluminum street light housing 400, and an electrically powered heat source in the form of a ballast or generator 402. To the generator 402 is attached a thermally conductive foam conduit 404. The thermally conductive foam conduit 404 is of porous, open-cell construction. Within the thermally conductive foam conduit 404 is placed an amount of thermally conductive particles 406. In some embodiments the exterior surfaces of the thermally conductive foam conduit 404 are coated or otherwise sealed, such as with a heat conductive epoxy, that restricts or otherwise limits the thermally conductive particles 406 from migrating within or from the thermally conductive foam conduit 404. In other embodiments, to the thermally conductive particles 406 may be applied an adhesive that restricts or otherwise limits the thermally conductive particles 406 from migrating from the thermally conductive foam conduit 404. In still other embodiments, the thermally conductive foam conduit 404 includes a thermally conductive barrier positioned to restrict or otherwise limit the thermally conductive particles 406 from migrating within or from the thermally conductive foam conduit 404.

By way of illustrative examples for the purposes of teaching and not restriction or limitation, the illustrated embodiment shows thermally conductive particles 406 comprising copper powder positioned within that half of the thermally conductive foam conduit 404 in thermal communication with the heat dissipating structure, shown here as the first or inner side 408 of the street light housing 400. In alternative embodiments other kinds of thermally conductive particles 406 are contemplated, such as but not limited to aluminum, iron, steel alloys, polymers, combinations thereof, and the like. Further, a thermally conductive foam conduit one-half inch (½″) thick may include a layer of thermally conductive particles 406 approximately one-quarter inch (¼″) thick, and a thermally conductive foam conduit one-quarter inch (½″) thick may include a layer of thermally conductive particles 406 approximately one-eighth inch (⅛″) thick. In alternative embodiments the layer of thermally conductive particles may be less than or greater than one-half the thickness of the thermally conductive foam conduit 404 The thermally conductive foam conduit 404 is then attached to the first or inner side 408 of the street light housing 400.

In this exemplary embodiment the heat source to be cooled is the generator 402. To the base of the generator 402 is attached a thermally conductive foam conduit 404 that includes a layer of thermally conductive particles 406. The thermally conductive foam conduit 404 with thermally conductive particles 406 is bonded with a heat conducting epoxy to the inner side 408 of the street light housing 400, and the base of the generator 402 is bonded with a heat conducting epoxy to the thermally conductive foam conduit 404, such that the thermally conductive foam conduit 404 is in thermal communication with the generator 402 and the inner side 408 of the street light housing 400. The street light housing 400 is in thermal communication with the thermally conductive foam conduit 404 from the inner side 408 and ambient air or atmosphere from the outer side 410.

Heat created by the generator 402 is transferred through its base to the thermally conductive foam conduit 404 and from the thermally conductive foam conduit 404 to the street light housing 400, which in the illustrated embodiment is constructed primarily of aluminum. The heat from the outer side 410 is then dissipated to the ambient air or atmosphere. An advantage of some embodiments according to the present disclosure is that the thermally conductive foam conduit 404 with the thermally conductive particles 406 can remove heat from the generator 402 at a faster rate than the thermally conductive foam conduit 404 without the thermally conductive particles 406. Accordingly, together with the thermally conductive foam conduit 404 and thermally conductive particles 406, the street light housing 100 constructed primarily of aluminum can dissipate greater heat and maintain a lower operating temperature than a stock or unmodified street light housing 400.

FIG. 5 is a chart that compares the operating or steady state temperature of a Cobra Head street light generator to various embodiments according to the present disclosure. One purpose for the test comparisons of FIG. 5 is to determine to what extent various embodiments of the present disclosure transfer heat from a heat source, such as a street light generator sealed inside a heat dissipating structure such as an aluminum street light housing, and to the ambient air or atmosphere. For each test illustrated in FIG. 5 the same Cobra Head street light, including the generator, lamp, and street light housing was used. For the tests the street light was modified as described below, and the tests were executed under an ambient or atmospheric temperature of 58 degrees Fahrenheit (58° F.). For the first test, labeled “Stock Light No Heat Conduit”, the stock generator was removed from the die cast aluminum stock street light housing and suspended inside the street light housing so as to provide an air gap in all directions between the generator and the inside of the street light housing. Aluminum comprises a density of 2.7 g/cc, a thermal conductivity of 181 W/mK, and a specific thermal conductivity of 67 W/mK/g/cc. For this first test the operating or steady state temperature of the generator reached 135° F. The air gap around all sides of the suspended generator accounts for the difference of 10° F. between the generator temperature illustrated in FIG. 2 and the generator temperature of this first test of FIG. 5.

For the second test, labeled “Modified Light L Type Carbon Heat Conduit”, a carbon foam conduit was placed in thermal communication between the generator and the inside of the aluminum street light housing, according to an embodiment of the present disclosure. Here, the carbon foam conduit designed “L Type” comprises a density of 0.38 g/cc, a thermal conductivity of 70 W/mK, and a specific thermal conductivity of 184 W/mK/g/cc. For this second test the operating or steady state temperature of the generator was reduced to 110° F. As compared to aluminum, from which the street light housing is constructed, the low density of the carbon foam conduit provides a thermal response which results in a lower steady state temperature for the generator.

For the third test, labeled “Modified Light P Type Carbon Heat Conduit”, a carbon foam conduit was placed in thermal communication between the generator and the inside of the aluminum street light housing, according to an embodiment of the present disclosure. Here, the carbon foam conduit designed “P Type” comprises a density of 0.7 g/cc, a thermal conductivity of 240 W/mK, and a specific thermal conductivity of 343 W/mK/g/cc. For this third test the operating or steady state temperature of the generator was reduced to 105° F. The higher thermal conductivity and low density of the P Type carbon foam conduit provides a thermal response which results in an even lower steady state temperature for the generator.

For the fourth test, labeled “Modified Light L Type Carbon Heat Conduit With Carbon Power”, copper powder was vibrated into the voids of the L Type carbon foam conduit used in the second test to provide a metallic layer inside the carbon foam conduit that substantially fills the voids at the surface interface between the aluminum street light housing and the carbon foam conduit. For this fourth test the operating or steady state temperature of the generator was reduced to 102° F. The carbon foam conduit has substantial surface area in a three-dimensional aspect but less surface area in a two-dimensional aspect. An embodiment that includes multiple types of layers in a passive heat exchanger can improve the passive heat exchange efficiency.

In many environments it may be desirable to increase the temperature or otherwise heat a component of a device, a device, or a system. Turning now to FIG. 6, there is illustrated an exemplary passive heat exchanger according to the present disclosure applied to a heat generation system. More specifically, the illustrated embodiment is a solar powered heat generation system 600 comprising a reflective parabolic trough 602 that receives and directs reflected sunlight 604 to a calculated focal point area 606. A channel having a fluid to be heated, such as a pipe 608 transferring water, is substantially encompassed by a thermally conductive foam layer 610 having voids 612 and placed substantially within the calculated focal point area 606. For the purposes of the present teaching and disclosure, the terms “foam conduit” and “foam layer” as used herein are synonymous and fully interchangeable.

In the illustrated embodiment the diameter of the focal point area 606 is calculated to be 1.125″ and the pipe 608 is a stock copper pipe, well known to those in the construction industry and plumbing trade, having an outside diameter of 1.125″. Alternative embodiments include other types, sizes, and schedules of pipe made of various material including iron, glass, polymers, combinations thereof, and the like. The thermally conductive foam layer 610 may be of any kind described herein or known, having voids 612 in the outer surface configured to receive heat. Here, the heat source is sunlight 604 and the element to be heated is water carried in a pipe 608. Still other embodiments include other types of elements, components, devices, and systems to which a thermally conductive foam layer 610 may be applied for the purposes of increasing temperature or heat, including heat pumps, thermal blankets, HVAC systems, systems having phase change materials, manufacturing processes, combinations thereof, and the like.

FIG. 7, with reference also to FIG. 6, is a chart that compares the thermal efficiency of various passive heat exchanger embodiments in the form of a solar powered heat generation system 600, according to the present disclosure. The thermal efficiency of the various passive heat exchanger embodiments is calculated by dividing the unit of Watts Transferred to the water flowing through the pipe 608 by the surface area of the trough perpendicular to the sunlight 604, to determine the Watts/Meter² collected. This result, labeled “Panel Watts/Meter²” is then divided by the measured energy available from the sunlight, the unit labeled “Sun Watts/Meter²”, to determine thermal efficiency labeled “Efficiency Panel/Sun”. The measure of “Efficient Panel/Sun” is a measure of the percent of energy available from the heat source that is transferred to the element to be heated, and the calculation is rounded up to the nearest whole integer. For the first test, labeled “Copper Water Pipe 1”, a stock copper pipe 608 having an outside diameter of 1.125″ and no thermally conductive foam layer 610 was positioned within the calculated focal point area 606 of the reflective trough 602. The temperature of the water flowing within the copper pipe 608 was heated by 7° F. and was approximately 19% efficient.

For the second test labeled “Carbon Outer Layer 2” a copper pipe 608, having an outside diameter of 0.75″ to which a carbon foam layer 610 of 0.125″ thickness is applied, was positioned within the calculated focal point area 606 of the reflective trough 602. This passive heat exchanger has an outside diameter of 1″. The temperature of the water flowing within the copper pipe 608 was heated by 9° F. and was approximately 37% efficient.

For the third test labeled “Carbon Outer Layer 3” a copper pipe 608, having an outside diameter of 1.125″ and coated with a carbon foam layer 610 was positioned within the calculated focal point area 606 of the reflective trough 602. The temperature of the water flowing within the copper pipe 608 was heated by 19° F. and was approximately 56% efficient. In this embodiment the carbon foam layer 610 is a thermally conductive foam in the form of a compound manufactured by grinding black carbon foam into powder and mixing the black carbon foam powder with an amount of epoxy sufficient to bond a layer of black carbon foam dust substantially around the circumference and length of the pipe 608. In one embodiment this compound comprises a first mixture of approximately 2 parts Aremco brand epoxy 631A mixed with approximately 2 parts black carbon foam powder and approximately 2 parts acetone, and a second mixture of approximately 2 parts Aremco brand epoxy 631B mixed with approximately 1 part black carbon foam powder and approximately 2 parts acetone. The first and second mixtures are then combined and applied to the element, device, component, or system to be heated or cooled and allowed to cure.

For the fourth test labeled “Carbon Outer Layer 4” a copper pipe 608, having an outside diameter of 1.75″ to which a carbon foam layer 610 of 0.125″ thickness is applied, was positioned within the calculated focal point area 606 of the reflective trough 602. This passive heat exchanger has an outside diameter of 2″. The temperature of the water flowing within the copper pipe 608 was heated by 14° F. and was approximately 40% efficient. For the fifth test labeled “Carbon Outer Layer 5” a copper pipe 608, having an outside diameter of 2.0″ to which a carbon foam layer 610 of 0.125″ thickness is applied, was positioned within the calculated focal point area 606 of the reflective trough 602. This passive heat exchanger has an outside diameter of 2.25″. The temperature of the water flowing within the copper pipe 608 was heated by 13° F. and was approximately 36% efficient. For the sixth test labeled “Carbon Outer Layer 6” a copper pipe 608, having an outside diameter of 0.5″ to which a carbon foam layer 610 of 0.5″ thickness is applied, was positioned within the calculated focal point area 606 of the reflective trough 602. This passive heat exchanger has an outside diameter of 1.5″. The temperature of the water flowing within the copper pipe 608 was heated by 37° F. and was approximately 77% efficient.

FIG. 8, with continued reference to FIG. 6, is a chart that compares the thermal efficiency of various passive heat exchanger embodiments, in the form of a solar powered heat generation system 600, according to the present disclosure. The first and second test results illustrated in FIG. 8 compare a passive heat exchanger embodiment having a carbon foam layer 610 on the inside of the copper pipe 608 and in contact with the water, to a passive heat exchanger embodiment having the carbon foam layer 610 on the outside of the copper pipe 608 and not in contact with the water. The third test compares a passive heat exchanger embodiment having a carbon foam layer 610 in the form of a tube, within a lexan tube and without a copper pipe 608, such that the water would flow through the carbon foam layer 610.

An advantage taught by the present disclosure is that a passive heat exchanger comprising a heat dissipater, such as a housing enclosing a component to be cooled or a channel carrying a fluid to be heated, in thermal communication with a carbon foam layer will more efficiently cool or heat than a heat dissipater without the carbon foam layer. Another advantage taught by the present disclosure is that a passive heat exchanger comprising a heat dissipater in combination with a carbon foam layer will more efficiently cool or heat than a passive heat exchanger without either the heat dissipater or carbon foam layer. Another advantage taught by the present disclosure is that an increase in the thickness of the carbon foam layer results in an increase in the efficiency of the passive heat exchanger. Still another advantage taught by the present disclosure is that the porosity of the carbon foam layer permits more energy to be trapped inside the voids of the carbon foam layer, energy that would otherwise be reflected away from the heat dissipater.

Embodiments of the present disclosure include methods of implementing a passive heat exchanger according to the present disclosure. Exemplary embodiments include methods of implementing a passive heat exchanger to reduce the temperature or otherwise cool an element of a device or system. Other exemplary embodiments include methods of implementing a passive heat exchange to increase the temperature or otherwise heat an element of a device or system. Some embodiments of various methods will now be described. It should be appreciated that more or fewer operations may be performed than described, that these operations may also be performed in a different order than the order described herein, and that the various operations described herein may be combined.

One routine of an exemplary method of implementing a passive heat exchanger according to the present disclosure begins with the operation of identifying a need for a passive heat exchanger. After identifying the need, for example, to cool one or more heat sources, one side or end of a carbon foam conduit is placed in thermal communication with the heat source. The next operation in this embodiment includes placing another side or end of the carbon foam conduit in thermal communication with a heat dissipater. A heat dissipating structure may include a thermally conductive housing that encloses the heat source, while a heat dissipating layer may include a thermally conductive heat sink, such as but not limited to a metallic plate in the form of a heat sink. The last operation in this embodiment is permitting the heat source to cool by dissipating heat to the carbon foam conduit and permitting the carbon foam conduit to dissipate heat to the heat dissipating structure or heat dissipating layer. In alternative embodiments, thermally conductive particles are located within the carbon foam conduit.

Another routine related to an exemplary method of implementing a passive heat exchanger according to the present disclosure begins with the operation of identifying a need for a passive heat exchanger. After identifying the need, for example, to heat one or more devices or components of a system, one side or end of a carbon foam conduit is placed in thermal communication with an element to be heated. The next operation in this embodiment includes placing another side or end of the carbon foam conduit in thermal communication with a heat source. The last operation in this embodiment is permitting the heat source to increase the temperature of the device or component by dissipating heat to the carbon foam conduit and permitting the carbon foam conduit to dissipate heat to the device or component. In alternative embodiments, thermally conductive particles are located within the carbon foam conduit.

The subject matter described herein is provided by way of illustration only and should not be construed as limiting. Various modifications and changes may be made to the subject matter described herein without following the exemplary embodiments and applications illustrated and described, and without departing from the scope of the present disclosure, which is set forth in the following claims. 

1. A passive heat exchanger system, comprising: a heat generating component; a heat dissipater proximate the heat generating component; and, a thermally conductive foam conduit, in thermal communication with the heat generating component and the heat dissipater, configured to transfer heat from the heat generating component to the heat dissipater.
 2. The system of claim 1, further comprising a heat sink in thermal communication with the heat dissipater and configured to transfer heat from the heat dissipater.
 3. The system of claim 1, wherein the thermally conductive foam comprises voids having thermally conductive particles positioned substantially throughout a first side of the thermally conductive foam.
 4. The system of claim 1, wherein the thermally conductive foam comprises a compound of thermally conductive foam powder, adhesive, and solvent.
 5. A passive heat exchanger system, comprising: a street light housing having a first side and a second side; a generator that produces heat positioned within the street light housing; and, a thermally conductive foam, in thermal communication with the generator and the first side of the street light housing, configured to transfer heat from the generator to the street light housing.
 6. The system of claim 5, further comprising a heat sink in thermal communication with the second side of the street light housing, configured to transfer heat from the street light housing to the atmosphere.
 7. The system of claim 5, wherein the thermally conductive foam comprises voids having thermally conductive particles positioned substantially throughout a first side of the thermally conductive foam.
 8. The system of claim 7, wherein the first side of the thermally conductive foam is in thermal communication with the first side of the street light housing.
 9. The system of claim 5, wherein the thermally conductive foam comprises a compound of thermally conductive foam powder, adhesive, and solvent.
 10. A method for transferring heat with a passive heat exchanger, comprising: identifying at least one heat source to be cooled; placing a first side of a thermally conductive foam in thermal communication with the heat source; and placing a second side of the thermally conductive foam in thermal communication with a heat dissipater, the thermally conductive foam configured to transfer heat from the heat source to the heat dissipater.
 11. The method of claim 10, further comprising allowing the thermally conductive foam to transfer heat from the heat source to the heat dissipater.
 12. The method of claim 10, further comprising positioning a heat sink in thermal communication with the heat dissipater, the heat sink configured to transfer heat from the heat dissipater.
 13. The method of claim 10, further comprising providing a thermally conductive foam having voids with thermally conductive particles positioned substantially throughout the one side of the thermally conductive foam.
 14. The method of claim 10, further comprising providing a thermally conductive foam comprising a compound of thermally conductive foam powder, adhesive, and solvent.
 15. A method for transferring heat with a passive heat exchanger, comprising: identifying at least one component to be heated; placing a first side of a thermally conductive foam in thermal communication with the component to be heated; and placing a second side of the thermally conductive foam in thermal communication with a heat source, the thermally conductive foam conduit configured to transfer heat from the heat source to the component to be heated.
 16. The method of claim 15, further comprising allowing the thermally conductive foam to transfer heat from the heat source to the component to be heated.
 17. The method of claim 15, further comprising providing a thermally conductive foam having voids with thermally conductive particles positioned substantially throughout one side of the thermally conductive foam.
 18. The method of claim 15, further comprising providing a thermally conductive foam comprising a compound of thermally conductive foam powder, adhesive and solvent.
 19. A passive heat exchanger device comprising substantially equal parts of a thermally conductive foam powder, adhesive, and solvent.
 20. The device of claim 19, wherein the thermally conductive foam powder is one of a carbon and graphite foam, the adhesive is epoxy, and the solvent is acetone. 